Draw the Lewis structure (including important resonance structures) of H2C2O4

Draw the Lewis structure (including important resonance structures) of H2C2O4 (oxalic acid). Circle all important resonance structures and label all formal charges.

The Correct Answer and Explanation is:

Lewis Structure of H₂C₂O₄ (Oxalic Acid)

Molecular formula: H₂C₂O₄
Structure: Oxalic acid contains two carboxylic acid groups: –COOH.
Its structure can be written as:

HOOC–COOH

Step 1: Count valence electrons

• H (2 atoms): 2 × 1 = 2 electrons
• C (2 atoms): 2 × 4 = 8 electrons
• O (4 atoms): 4 × 6 = 24 electrons
  Total valence electrons = 2 + 8 + 24 = 34 electrons

Step 2: Draw the basic skeleton

Attach atoms in this way:

scssCopyEditHO–C(=O)–C(=O)–OH

Each carbon is bonded to one hydroxyl group (–OH) and double bonded to another oxygen atom.

Step 3: Add electrons to satisfy the octet rule

Structure 1 (most stable):

mathematicaCopyEdit      O        O
      ||       ||
H–O–C–––C–O–H

This shows both carboxylic acid groups with C=O and O–H bonds.

• Each O in C=O has 2 lone pairs
• Each hydroxyl O has 2 lone pairs
• No formal charges

Step 4: Resonance structures

In each carboxyl group, the negative charge can be delocalized between the two O atoms after deprotonation (loss of H⁺). So for the deprotonated form, resonance occurs:

Structure 2 (after deprotonation of one –OH):

mathematicaCopyEdit      O⁻       O
       |        ||
H–O–C–––C–O–H   <==>   H–O–C=O  ↔  H–O–C⁻–O

• The C–O⁻ bond can form a double bond while the C=O bond becomes C–O⁻
• This gives two resonance forms for each carboxylate group

Formal Charges (in resonance forms)

• On O⁻: –1 formal charge (6 valence e⁻ – [6 non-bonding + 1 bonding e⁻]) = –1
• All other atoms in the neutral form have formal charges of 0

Summary:

• The neutral H₂C₂O₄ has no formal charges in the fully protonated structure
• Resonance occurs in the deprotonated forms (common in acid-base reactions)
• Important resonance structures are those with delocalization of negative charge in carboxylate ions

Note: You should draw the two carboxyl groups with delocalized electrons when discussing the conjugate base. Circle those with delocalized negative charges as the important resonance structures.